As consumers become more entrenched in their digital lives, companies are finding more and more applications for including heart rate monitors in wearable devices. Applications ranging from fitness to healthcare, gaming to first responders are finding the accuracy and flexibility of an optical heart rate monitor to be ideal.
Click to download: OHRM body location diagram
The location of the device, and therefore the heart rate sensor system, on the body can have a huge impact on the accuracy of the device. An accurate heart rate monitor can deliver not only heart rate, but many other derivative metrics such as VO2 max, cardiac efficiency, heart rate recovery, and fatigue.
For those looking to add heart rate monitor to a wearable device, it’s important to understand before you design your product just where on the body you will get the most accurate readings.
Why does location matter? Because accuracy matters. Accurate readings can be make or break for wearable devices because if a consumer can’t trust the readings from the device, the user experience turns negative very quickly. This can’t lead to product returns, lost customers, and brand damage.
While we won’t go into a detailed explanation of heart rate monitors (read What you need to know for that), it is important to understand where and why certain parts of the body are more appropriate for heart rate monitor wearables than others. Heart rate monitor accuracy is dependent upon the ability to measure light scattered from flowing blood. Locations that increase light scattering limit the accuracy, which limits the number of metrics and use cases that you can support for your customers.
Physiology and Technology
Because optical heart rate monitors work by shining light into the body and measuring how light is scattered from blood flow, the technology works best in areas of the body that limit the amount of light that is scattered or absorbed by physiological characteristics that are not related to blood flow. Accuracy may be compromised when trying to gather data from areas of the body that have a diversity of tissues, such as bone, muscle, tendons, etc. It’s also negatively impacted by parts of the body that experience more movement when the body is in motion, such as wrists and ankles.
Here, in relative order of accurate measurement, are the locations on the body where heart rate monitors on wearables are able to gather the best heart rates:
Pros: The ear is stable, with very limited movement even when the body is in motion. Also, because it is mainly cartilage and arterioles, there are fewer disturbance factors. The ear boasts an ideal arteriole bank between the anti-tragus and concha of the ear; since this arteriole bank is directly connected to the carotid artery system, blood flow characteristics measured at this location can provide a true picture of what’s going on in the heart. Moreover, this region of the ear does not interfere with the ear canal or audio electronics, and it is more uniform across a larger population than other ear locations, making earbud design more flexible. Add to that the comfort level consumers already have with earbuds, and this location ranks highly.
Cons: Exposure to “environmental noise” – sunlight, in this case – is challenging, especially when shadows are present. No two ears are shaped the same, making it difficult to get a proper universal fit without multiple gel sizes available. And finally, wearing a device in your ear 24×7 is not currently socially acceptable outside of hearing aids.
Mitigation: Three things can be done to temper these concerns.
- Use infrared light to alleviate sunlight issues;
- Use active signal characterization, which works much like noise cancellation, to filter out environmental impactors;
- Use proven designs that enable consistent sensor placement in ears of all shapes and sizes to encourage an accurate read.
Pros: Very good place to measure heart rate, as there is little relative motion noise and a clean signal can be acquired. It’s also a great spot for integration within helmets, headbands, and other head-worn apparel.
Cons: Wearing a heart rate monitor during daily life activities on your forehead isn’t exactly a fashion statement, and in some cases could be a hindrance, so in certain applications, adoption is questionable or impossible. Also, because it is open to the sun, there may be problems getting enough sunlight rejection in the form factor.
Mitigation: This is where design matters. If outstanding accuracy is your goal, embed the wearable in a headband, a hat or even VR goggles.
Pros: People are comfortable wearing things on their wrists, and are predisposed to think this is a good location because that’s where they take their pulse.
Cons: The sheer number of bones, tendons, and muscle create higher optical noise in the wrist. Pair with that the high degree of variability in vascular structure, skin tone and blood perfusion across populations, and the wrist becomes one of the most difficult places on the body to measure heart rate accurately.
Mitigation: Active signal characterization identifies motion noise and reduces obstructive concerns, and certain designs can provide enough shadow to mitigate environmental light, making the write more likely to produce accurate readings. Using additional wavelengths can help overcome the skin tone limitation so that both light and dark-skinned users can enjoy the same level of accuracy.
Pros: Large amount of blood flow due to the large muscles in the area.
Cons: More relative movement than the head; unlike earbuds and wrist-worn devices, very few existing consumer products are worn at the forearm, and thus wearing a forearm band may not be as socially acceptable.
Pros: Like the arm, the calf has high blood flow.
Cons: Activities like running and walking create a shock force that makes accurate readings harder. Plus, people generally don’t want to wear something strapped to their leg.
Pros: The most positive reason to use the ankle is its inconspicuous location.
Cons: The ankle is filled with tendons and ligaments, and generally has very limited blood flow. The ankle has some of the highest intensity of motion artifacts of any location of the body.
Mitigation for the arm, calf, and ankle: You must rely primarily on testing. Test thoroughly and often, on many people of all shapes, sizes, skin tones and fitness levels doing things that you expect them to do when they buy the device.
Implications for Wearables: Design for the objective
As there are pros and cons to optical heart rate monitoring on different parts of the body, the decision about where your new device should rest is highly dependent on the use case and the market for the product.
Military and first responders will have more restrictions on excess accouterment, yet they’re more likely to be required to wear a helmet or radio earbud which provides greater access to the forehead or ear. Gamers, on the other hand, will respond to their competitive nature, likely with less regard for fashion and more appreciation for accuracy.
The right location for the right task
No matter the design objective or use case for the wearable, data accuracy is critical to the success of any wearable device. If you’ve chosen to utilize a heart rate monitor in your next device, it’s important that you keep in mind the different levels of performance accuracy you can expect at different locations on the body.
There are different mitigation strategies for the issues that will be encountered at different body locations. And while application of critical methodologies, such as active signal characterization, can be a boost, validation, and iteration through comprehensive testing are critical to success.